A Quick Tour of the Universe

(and this course)Part 2

Charting the Heavens: Foundations of Astronomy

Learning Goals• Describe the Celestial Sphere and how astronomers use

angular measurement to locate objects in the night sky.

• Account for the apparent motions of the Sun and the stars in terms of the actual motion of the Earth. Explain why our planet has seasons.

• Understand the changing appearance of the Moon and how the relative motions of the Earth, the Sun, and the Moon lead to eclipses.

Clicker Question:

If Earth rotated twice as fast as it currently does, but its motion around the sun stayed the same, then which of the following is true:

A: the night would be twice as long

B: the night would be half as long

C: the year would be half as long

D: the year would be twice as long

E: the length of a day would be unchanged

Clicker Question:

If Earth rotated twice as fast as it currently does, but its motion around the sun stayed the same, then which of the following is true:

B: the night would be half as long

Clicker Question:

Distances to nearby stars are measured by:

A: bouncing radar signals off them

B: using laser beams

C: using geometry and parallax measurements

D: measuring how long it took a spacecraft to get there and back and assuming a constant velocity.

Clicker Question:

Distances to nearby stars are measured by:

C: using geometry and parallax measurements

Clicker Question:

How many stars are there in the observable universe?

A: 1012

B: 1022

C: 1032

D: 1042

E: infinite

Clicker Question:

How many stars are there in the observable universe?

B: 1022

The Sky at Night

What do we see?

The MoonPlanetsPerhaps a meteor shower, comet, or other rare eventStars - about 3000 visiblePatterns of stars - constellations 88 of them Useful for finding our way around the sky, navigating the oceansSatellites, airplanes, clouds, lightning, light pollution ...

The Celestial Sphere

Features:- Does not rotate with Earth- Poles, Equator- Coordinate System

An ancient concept, as if all objects at same distance.

But to find things on sky, don't need to know their distance, so still useful today.

The "Solar Day" and the "Sidereal Day"

Solar Day

How long it takes for the Sun to return to the same position in the sky (24 hours).

Sidereal Day

How long it takes for the Earth to rotate 360o on its axis.

These are not the same!

One solar day later, the Earth has rotated slightly more than 360o .A solar day is longer than a sidereal day by 3.9 minutes(24 hours vs. 23 hours 56 minutes 4.091 seconds).

Clicker Question:

Why is it warmer in Albuquerque in the summer than winter?

A: The northern hemisphere is tilted towards the sun in summer.

B: The Earth is closer to the sun in summer.

C: The greenhouse effect increases in summer.

D: The sun increases its intrinsic luminosity in the summer.

E. All of the above.

Clicker Question:

Why is it warmer in Albuquerque in the summer than winter?

A: The northern hemisphere is tilted towards the sun in summer.

The Earth's rotation axis is tilted with respect to its orbit around the Sun => seasons.

Summer Winter

Scorpius Orion

Tilt is 23.5o

DayNight Day NightSun high in northern sky

Sun low in northern sky

Winter

In winter, sunlight is spread out more thinly across the ground => each bit of ground receives less radiation => cooler

Summer

Summer Winter

ScorpiusOrionDayNight Day Night

The Year

The Earth revolves around the Sun in 365.256 days (“sidereal year”). But the year we use is 365.242 days (“tropical year”). Why?

Precession

The Earth has a bulge. The Moon "pulls down" on the side of the bulge closest to it, causing the Earth to wobble on its axis (how do we know this?)

Earth

Moon

Spin axis

**Vega Polaris

Precession Period 26,000 years!

Precessionanimation

The Motion of the Moon

DEMO - Precession

Winter: July or January?

Winter: January

Scorpius OrionDayNight Day Night

Scorpius OrionNightDayNight Day

Summer: January or July?

Summer: July

Now

13,000 years from now

We choose to keep July a summer month, but then in 13,000 years, summer occurs on other side of orbit!

The Motion of the Moon

The Moon has a cycle of "phases", which lasts about 29 days.

Half of the Moon's surface is lit by the Sun.

During this cycle, we see different fractions of the sunlit side.

Which way is the Sun here?

See Tutorial on book’s websitefor animation.

The Motion of the Moon

DEMO - Phases of the Moon

Cycle of phases slightly longer than time it takes Moon to do a complete orbit around Earth.

Cycle of phases or "synodic month"

Orbit time or "sidereal month"

29.5 days 27.3 days

Eclipses

Lunar Eclipse

When the Earth passes directly between the Sun and the Moon.

Sun Earth Moon

Solar Eclipse

When the Moon passes directly between the Sun and the Earth.

Sun EarthMoon

Solar Eclipses

Total

Diamond ring effect - just before or after total

Partial

Annular - why do these occur?

Lunar Eclipse

Why don't we get eclipses every month?

Moon's orbit tilted compared to Earth-Sun orbital plane:

SunEarthMoon

Moon's orbit slightly elliptical:

Earth

Moon

Side view

Top view, exaggerated ellipse

Distance varies by ~12%

5.2o

Types of Solar Eclipses Explained

Certain seasons are more likely to have eclipses. Solar “eclipse season” lasts about 38 days. Likely to get at least a partial eclipse somewhere. Animation

It's worse than this! The plane of the Moon's orbit precesses, so that the eclipse season occurs about 19 days earlier each year.

Recent and upcoming total and annular solar eclipses

Eratosthenes Determines the Size of the Earth in about 200 B.C.

SyeneAlexandria

Sun's rays

7.2o S

N

Earth

He knows the distance between the two cities is 5000 "stadia".

From geometry then,

7.2o

360o Earth's circumference

5000 stadia =

=> circumference is 250,000 stadia, or 40,000 km.

So radius is:

40,000 km

2 pi = 6366 km

(very close to modern value, 6378 km!)